In the Garden:New England
December, 2010
Regional Report

Oak Leaves Just Keep on Hangin'On

A couple of decades ago, when I planted a red oak (Quercus rubra) to begin to fill the blank canvas that was the front yard of my newly-built house, I was thinking about how well adapted the tree was to my acid, sandy soil, how majestic it would look as it matured, how its acorns would provide food for squirrels and blue jays, how its deep roots would make it easy to plant a shade garden beneath its spreading limbs.

All of which came to pass. But what I didn't think about was how this tree would hang on tightly to its leaves in fall, that they'd rattle in tatty bundles in the winter wind, and that many would still be clinging to branches even as the new leaves were coming out in spring.

So, although my oak is indeed a magnificent tree that has flourished where it was planted, I sometimes wish I'd chosen a tree that politely and neatly dropped all its leaves at once by mid-fall. So why does my oak insist on keeping its dead and dried leaves attached?

In order to shed their leaves for the winter, most deciduous trees, in response to the decreasing daylengths and lower temperatures of fall, produce an enzyme that causes what is called an abscission layer to form where the leaf petioles attach to a branch. As the cell walls in this specialized layer break down, it only takes a few fall breezes to cause the leaves to detach and fall to the ground.

However, certain species, including oaks, beeches and hornbeams, called "marcescent" species, either fail to form an abscission layer, wait until late in winter to form it, or begin forming the specialized cells but don't complete their development until spring. In any case, the leaves remain firmly on the tree even after they are dried and brown. Young trees and the lower branches that comprise the most juvenile wood on older trees are most likely to hold onto their leaves. So my 20 year old tree, which is beginning to approach young adulthood in oak terms, holds the leaves on its lowest branches the longest.

No one really knows why some species fail to shed their leaves after they're no longer serving a photosynthetic function for the tree, but it's been suggested that these unpalatable clusters of dead leaves may keep the tender new spring buds from being browsed by hungry herbivores like deer and moose. Which is all well and good, but I still wish I didn't have to look at dead leaves all winter and finish my "fall" raking in the spring!